Summary: The overall objectives of this proposal is to study the mechanism whereby transcobalamin II facilitates the uptake of vitamin B-12 by the cell. Initially, we will use the He La cell as a model system (1) to study the biochemical events leading to the penetration of B-12 into the cell. One approach is to label the TC II with I125 and the B-12 moiety with Co57. Exposure of the doubly labeled complex to HeLa cells should provide information as to whether the intact complex penetrates the cell, or whether dissociation of the B-12-TC II complex occurs prior to the entry of B-12 into the cell. No change in the I125/Co57 ratio would indicate that the entire complex became associated with the cells, however, a lowering of the ratio would indicate that only the B-12 portion of the complex remained associated with the cells. We also intend to use I125 labeled TC II in an attempt to determine whether or not HeLa cells contain a specific receptor for TC II on the cell surface. If in vivo experiments prove successful, we will isolate plasma membrane from HeLa cells to study the interaction of the TC II-B12 complex with the membrane in a more simplified system. If successful, other tissues and cell types would be tested for specific TC II receptor sites.